This invention relates generally to a nonwoven fabric or web which is formed from fine spunbond fibers of a thermoplastic resin and the process of producing such a fabric.
Thermoplastic resins have been extruded to form fibers and webs for a number of years. The most common thermoplastics for this application are polyolefins, particularly polypropylene. Each material has its characteristic advantages and disadvantages visa vis the properties desired in the final product to be made from such fibers.
Nonwoven fabrics are useful for a wide variety of applications such as diapers, feminine hygiene products, incontinence products, towels, medical garments and many others. The nonwoven fabrics used in these applications are often in the form of laminates like spunbond/meltblown/spunbond (SMS) laminates. In SMS laminates the exterior layers are spunbond polypropylene which are usually present for strength, and the interior layer is meltblown polypropylene which is usually a barrier layer.
One of the desirable characteristics of fibers used to make the nonwoven fabrics is that they be as fine as possible. Finer fibers are desired in many applications where smoothness and uniformity are important such as, for example, in infant care products like diapers. Fine fibers are also desired since they provide better area coverage for a given amount of polymer and they allow the use of fabrics of lower basis weights. Basis weight is a measurement of area coverage and is measured in ounces per square yard (osy) or grams per square meter (gsm) of fabric. Lowering the basis weight would make products made from these fabrics lighter, thinner and, assuming equal cost per basis weight, less expensive than those made from conventional fibers.
Finer fibers, while desirable, have the drawback of generally being weaker than larger diameter fibers. This effectively sets a lower limit on the basis weight of the fabric which can be made from the fibers because of processing problems in fiberization and fabric production or because of lack of strength of the resulting fabric. The industry is constantly searching for a fiber which is very fine but which retains as much of the strength of conventional spunbond material as possible.
It is an object of this invention to provide fibers which are finer than those conventionally produced and which have comparable strength characteristics.